Holley fine-tuning problems

[Dennis O.]

After several years, I have the 4160 Holley on my '67 L79 running very well. It starts at the touch of the key, idles well, and all around runs out pretty good. It does, however, have what I believe is called a "lean stumble" under certain circumstances. What happens is that when I am cruising around on the pleasant country roads up here in the lake district, I will occasionally encounter a slight upgrade. Every once in a while the car will miss a beat, but then take up where it left off and continue on. I have never encountered any problems like this when hitting the throttle under other circumstances, such as passing, accelerating out of a 30 mph zone, etc. It seems to happen only when the load increases on the engine slightly with no change in throttle. I immediately suspected the power valve. I have an aftermarket Holley technical book the tells how to select a power valve for any particular engine using a vacuum gauge. I hooked up a known good vacuum gauge and went for a long drive. here are the results from a completely warmed up engine :

Idle - 15' hg
35 mph - 17.5' hg
55 mph - 17.5' hg
hard acceleration - 5' hg
mild acceleration - 10' hg

According to the book, the proper power valve for this engine would be a 8.5. I installed a new one and encountered this problem. I replaced the 8.5 with a 6.5 and there was no difference. I then replaced the 6.5 with a 9.5, and still no difference. Everything I read (in the book and on here) seems to indicate that I need a higher number power valve. Since the 9.5 is the highest available I can find, I'm at an end.

I am starting to think this may not be a power valve problem. The engine is a fresh build (2700 mile) 327 with 9.7:1 pistons and a Comp Cams 262H cam and stock ignition. As I said above, it starts and runs excellently, seems to make plenty of power and runs at a steady 180 degrees. The timing seems to be right on. The carburetor has stock 61 jets (new this year). I have been reading the "ported vs manifold" vacuum threads, but they seem to apply only to big blocks. Perhaps I need larger jets, a new vacuum can, or whatever?

I am asking for a little guidance here as I am afraid I'll wear the carb out taking it apart so many times. (I have already had to replace one of the "o"-rings on the fuel transfer tube.)

Any suggestions?

[Ken A.]

Re: Holley fine-tuning problems

Stumble under load points to weak coil or bad SP wires. Put back the 65 power valve.

[Clem Z.]

Re: Holley fine-tuning problems

make sure the throttle arm in not loose on the shaft.

[Jim T.]

Re: Holley fine-tuning problems

I agree with Ken, use the 6.5 power valve. That is what I use in the Holley on my L79.

[William C.]

Re: Holley fine-tuning problems

Look hard at the components of the ignition system, and give some thought to upping the primary Jet size by a couple of thou.. If you can't repeat the problem, it might have been just a random event, but if it repeats it needs to be corrected.

[Dennis O.]

Re: Holley fine-tuning problems

Went on an extended cruise today; the 9.5 power valve seems to have made things worse. I am going back to the 8.5 and putting in 63 jets; stay tuned.

[Timothy B.]

Re: Holley fine-tuning problems

Dennis,

The stock 600 Holley uses a #65 primary main jet, the #61 may be too lean. On a flat road at cruise at 35-45-55 mph, does the engine surge as this would indicate to lean primary jet. The power valve selection IMO does not have anything to do with the problem, by installing a higher # valve your simply enriching the mixture sooner and at worst your gasoline mileage will suffer.

Make sure the accellerator pump is adjusted correctly and when the front bowl is off double check the adjustment of the accellerator pump check ball so there is only .010 +- between ball and retainer as this may be the cause of the hesitation. When you lay your foot on the pedal the first bit of pump shot pressure is wasted seating the check ball if it's not adjusted properly.

[Gerard F.]

Re: Holley fine-tuning problems

Dennis,

Maybe try playing around with the timing and dwell. A higher dwell would tend to retard the timing.

I had a problem with my stock 67/300 going up a hill to my house in first. It would stumble and sometimes stall.

A friend did a simple adjustment of the timing to spec with the dwell at the lower range. Solved the problem.

Maybe worth a try, see where you are at in dwell, lower it a bit and try it out.

[Dennis O.]

Re: Holley fine-tuning problems

Lots of things nto try here. I think the first thing I'm going to do is put everything back to factory and start from scratch. Timing, jets, power valve, timing, dwell, etc. I wouldn't be surprised if just that solves my problem. I think I got too "fancy" selecting my power valve.

[Chris H.]

Re: Holley fine-tuning problems

I vote for the accelerator pump. Make sure the hinge is well lubricated and the squirters aren't plugged.

[Gordon W.]

Re: Holley fine-tuning problems

Chris Hewitt, I was thinking squirters until I re-read the original post. He's talking about not moving the throttle at all, but something like going up a hill, so the accelerator pump and squirters should be a non-issue.

[Dennis O.]

Re: Holley fine-tuning problems

I always like it when someone who posts a problem comes back in the end and tells what the solution was. On this one, after I returned everything to the factory settings, I still had an occasional stumble on slight acceleration. After the last unsuccesful try, I rechecked all of the settings, and everything was right on. Since I have been having so many problems, I got lazy and left the air cleaner off. I took it out for a test, and the stumble was gone; the car ran and drove perfectly. I gleefully returned to my shop and called it a day. The next morning, I went back, put the aircleaner back on and went for another test drive; crap! the stumble was back.

Long story short, in one of the many times I have had the carburetor off the car, I must have lost or misplaced the gasket that goes between the carb and the air cleaner, and it wasn't on the car. I went to my stash of parts, got one out and installed it on the car. Problem solved, All I can figure is that there is minimal clearance between the air cleaner base and the choke linkage that was causing the problem when the gasket was not installed, and putting the gasket back on lifted the air cleaner enough to solve the problem.

I always thought that gasket was just an extra seal that wasn't really needed, but apparently not; anyway, I am a happy camper now; I've put over 100 miles on the car since adding the gasket and all seems to be perfect. Sttrange but true.

[Gordon W.]

Re: Holley fine-tuning problems

Wow! I always thought the same as you regarding that gasket.

[Morris R.]

Re: Holley fine-tuning problems

I always like it when someone who posts a problem comes back in the end and tells what the solution was. On this one, after I returned everything to the factory settings, I still had an occasional stumble on slight acceleration. After the last unsuccesful try, I rechecked all of the settings, and everything was right on. Since I have been having so many problems, I got lazy and left the air cleaner off. I took it out for a test, and the stumble was gone; the car ran and drove perfectly. I gleefully returned to my shop and called it a day. The next morning, I went back, put the aircleaner back on and went for another test drive; crap! the stumble was back.


Long story short, in one of the many times I have had the carburetor off the car, I must have lost or misplaced the gasket that goes between the carb and the air cleaner, and it wasn't on the car. I went to my stash of parts, got one out and installed it on the car. Problem solved, All I can figure is that there is minimal clearance between the air cleaner base and the choke linkage that was causing the problem when the gasket was not installed, and putting the gasket back on lifted the air cleaner enough to solve the problem.

I always thought that gasket was just an extra seal that wasn't really needed, but apparently not; anyway, I am a happy camper now; I've put over 100 miles on the car since adding the gasket and all seems to be perfect. Sttrange but true.

Hi Dennis,
Does a 66 have a gasket also? I have no idea what that is. Please inform. Thank you

[Dennis O.]

Re: Holley fine-tuning problems

If it has a Holley carb, I'm sure it does. It is Holley part # 108-4 according to the package mine came in. (The -4 part may indicate that they come 4 in a package.)